1. Field of the Invention
The present invention relates to a nitrogen oxide detecting sensor and a method of manufacturing the sensor and more particularly to a nitrogen oxygen sensor to be used in the art of reducing or decomposing nitrogen oxide and a method of manufacturing such sensor.
2. Description of the Related Art
As the conventional methods of measuring a concentration of nitrogen oxide in exhaust gas, there are known the chemiluminescence method, infrared absorbing analysis method, ultraviolet ray absorbing analysis method, controlled potential electrolysis method and controlled potential coulomb method.
Further, in recent years, there is a growing scientific interest to employ, as an oxide semiconductor sensor, a compound expressed by: EQU YBa.sub.2 Cu.sub.3 O.sub.7-.delta.
At present, the chemiluminescence method is considered as the most promising of all for its highest measurement precision and reliability. However, this method requires such components as an ozone generator, a photomultiplier tube, and a high voltage source. Thus, this method suffers from limitations in the degrees of system size reduction, cost reduction and maintenance conditions that are possible.
When high detection precision is not needed, the controlled potential electrolysis method is convenient. This method, however, suffers from the problems of secular or long-term changes in the electrodes and of having to maintain the electrolysis solution.
Moreover, the devices employed by these methods cost hundreds or thousands of dollars. Also, while the measurement precisions are high, the methods require calibration due to the occurrence of drift and the devices employed have the problem of durability.
On the other hand, the above-described oxide semiconductor sensor has the problem that its sensitivity characteristics are subject to change if water is present in the detection environment.
Further, it is also known that an oxide compound having a composition of: Bi:Sr:(Ca.sub.1-x Y.sub.x):Cu=2:2:1:2 is subject to changes in its resistance value due to the presence of NO or NO.sub.2, and therefore can be used as a gas detecting portion of a sensor. However, even with this sensor having such an oxide compound, if the sensor is exposed to a high concentration of nitrogen dioxide (NO.sub.2), the resistance value at the gas detecting portion remains raised, not returning to the lower original value. Hence, in the detecting portion, there occurs drifting in the base resistance value, i.e., the zero reference point, due to aging such that the sensor becomes unable to provide its sensor function. For this reason, in spite of its superior selectivity to NO.sub.x, the above-described composite oxide having a composition of: Bi:Sr:(Ca.sub.1-x Y.sub.x):Cu=2:2:1:2 has not been put into practice.
Accordingly, a primary object of the present invention is to provide a nitrogen oxide sensor that is simple in its construction, economical and durable. In addition, the primary object is to provide a method of manufacturing such a sensor.
A further object of the invention is to provide a sensor having superior selectivity to NO.sub.x and comprising an oxide compound that has the composition: Bi:Si(Ca.sub.1-x Y.sub.x):Cu=2:2:1:2, the compound including a 2212 phase crystalline structure at more than a predetermined ratio, such that when exposed to a high concentration of NO.sub.x, the sensor is capable of immediately returning to its original resistance value, i.e., its zero point when exposed later to a gas not containing NO.sub.x.